1. Field of the Invention
This invention relates to a cable, comprising one or more optical fibers, that is at least partially filled with a waterproofing compound.
2. Description of the Prior Art
Optical fiber cables typically include a sheath that is designed to surround and protect one or more optical fibers located therein. One type of optical fiber cable construction is shown in U.S. Pat. No. 4,241,979, coassigned with the present invention. In that cable, one or more optical fibers are placed within an inner sheath. A space between the fibers and the sheath prevents longitudinal forces from the sheath from substantially acting on the fibers. Various other cable designs are proposed, or in use, for optical fiber cables, including designs wherein the fibers are arranged in a groove surrounding a central strength member, among others.
To prevent water entry into cables, there has come into use various filling compounds for filling the voids within a typical cable. Filling materials have included petroleum jelly mixed with a polymer, usually polyethylene, as described in U.S. Pat. Nos. 3,607,487 and 3,717,716. Other filling materials have included a low viscosity oil gelled by a styrene-isoprene-styrene copolymer, again with polyethylene added to impart consistency and reduce slump; see, for example, U.S. Pat. No. 3,879,575. Most recently, an improved filling material having superior handling characteristics comprising a styrene-ethylene-butylene-styrene block copolymer and mineral oil with polyethylene added for consistency has been described in U.S. Pat. No. 4,176,240, coassigned with the present invention.
While the above-noted filling material has proved satisfactory in many cases, special attention to the peculiar characteristics of optical fibers requires that in some cases new filling materials are desirable. In particular, optical fibers have a requirement that microbending loss be minimized. Microbending losses increase the attenuation of the optical radiation in the fibers, limiting the length of transmission in a fiber; see, for example, Gardner, "Microbending Loss in Optical Fibers," Bell System Technical Journal, Vol. 54, No. 2, pages 457-465 (1975).
In the case of filled cables, a temperature change that causes a large compressive strain on the fibers due to the coupling with the filling compound can increase microbending loss. Therefore, it is desirable that the filling material have a low modulus to reduce the resulting strain on an optical fiber coupled thereto. However, the other desirable characteristics of a filling material should be maintained. For example, the material should not slump excessively at elevated service temperatures, so that the material will not migrate out of the end of the cable. The material should not be oily or greasy, but should have a soft-rubber texture to allow for convenient handling during cable repair and other operations. Furthermore, the viscosity of the material should be sufficiently low so that the cable can be filled rapidly with the filling material during manufacture of the cable.